The invention relates to an apparatus for measuring performance characteristics of lenses. More particularly, the invention concerns an apparatus for testing the image quality of a lens using an improved modulation transfer function (MTF) test system.
Electronic methods have largely replaced visual methods, such as resolving power, for testing lens in the industry. Presently many in the photographic and optical industries custom build electronic systems, typically either analog or digital modulation transfer function (MTF) designs, for testing lenses being produced in high volume. These lenses have thus far been manufactured with fixed, predetermined magnifications.
Modulation transfer function (MTF) design systems are characterized by a graphical representation of image contrast relative to the pattern contrast over a range of spatial frequencies, where high frequency in the test pattern corresponds to small detail in an object. As shown in FIG. 1, existing MTF design systems 1 typically include the following major elements: the test pattern arranged at the object plane 2; the lens 4 under test; and, the displaceable detector 40 (displacment noted by arrow) at the image plane 8. An important advantage of MTF design systems is that they provide information about image quality over a range of frequencies rather than just at the limiting frequency as does other conventional methods, such as resolving power. For many fixed magnification lenses, these test patterns are placed at fixed positions in the object plane. Moreover, the long conjugate distance d, i.e., the distance from the object plane 2 to the lens 4 shown FIGS. 1, 3 and 4, are also predetermined or fixed with fixed magnification lens.
Referring to FIG. 2, illustrated is a schematic diagram of a typical optical test pattern 10 imaged at the object plane (not shown). The test pattern 10 is imaged by the particular lens under test on the detector (not shown) at the image plane. The detector 40, preferably a charge coupled device (CCD), is generally moved in the direction of the optical axis and the image is analyzed in terms of modulation transfer function (MTF) as a measure of image quality. It should be appreciated that for a fixed magnification lens to be tested, such as wide angle and telephoto lenses, test patterns are placed at fixed positions in the object plane.
Referring to FIG. 3, a schematic diagram of a prior art wide-angle, fixed magnification lens is illustrated. Wide angle lens typically have a field angle, i.e. the angle between the axis test pattern and the field test pattern, between 15 and 20 degrees. Two things in the test system generally need to change when the lens changes from wide to telephoto. First, the field angle needs to change because the lens will have different fields of view from wide to telephoto. Those skilled in the art will appreciate the desire to want to test the lens at some percentage of the total field. Second, the spatial frequency of the test pattern must change to account for the different magnification when the lens goes from wide to telephoto.
Referring to FIG. 4, a schematic diagram of a prior art telephoto, fixed magnification lens with typical field angles of 4 to 6 degrees is illustrated.
Skilled artisans will appreciate that for zoom lenses a different set of test criteria must be employed. This is because the more versatile zoom lens has a variable focal length that must be tested at more that one zoom setting. Thus, the position of the test patterns in the MTF design system need to change to accommodate the changing field of view. Further, the test pattern spatial frequency also needs to change to keep the spatial frequency at the image plane appropriate for the lens and the detector.
Accordingly, a major shortcoming of current apparatus for testing image capabilities of lenses, such as zoom lenses that are to be tested at more than one zoom setting, is the position of the test patterns needed to change to accommodate the changing field of view. Moreover, another problem associated with present systems for testing such lenses is that the test pattern spatial frequency also needs to change to keep the spatial frequency at the image plane appropriate for the lens and the detector.
Therefore, a need persists for an apparatus for testing lenses that incorporate improvements to the digital system to accommodate the testing of lenses having variable focal lengths.
It is, therefore, an object of the invention to provide an apparatus that can vary the spatial test frequency and field angle for a lens being tested.
It is another object of the invention to provide an apparatus that permits infinitely variable field angles of the lens being tested.
Yet another object of the invention is to provide an apparatus that uses a reflecting surface arranged in a predetermined optical path which translates and rotates so the field angle of the lens being tested can change and still use the same test pattern system.
It is a feature of the invention that a flexibly mounted reflecting surface and a rotatable support plate containing a plurality of test patterns are arranged in a predetermined optical path contains a plurality of test patterns each having a distinct single spatial frequency for imaging by the lens being tested.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, a variable field angle and test pattern spatial frequency optical assemblage for testing a lens, said lens having a plurality of field angles characteristic of the magnification of the lens being tested, said apparatus comprising:
a source of illumination for emitting light;
a rotatable platform for supporting a plurality of spatially separated test patterns, said rotatable platform being capable of rotatably positioning any one of said plurality of test patterns in a predetermined optical path for varying said spatial frequency of said test pattern;
a collimating lens arranged in said predetermined optical path for receiving light transmitted through said any one of said plurality of test patterns and converting said light to a collimated array of light rays;
a reflecting surface arranged in said predetermined optical path for receiving said collimated array of light rays, said reflecting surface being capable of translational and rotational movements about an axis defined by said predetermined optical path for reflecting and then directing the collimated array of light rays through said lens being tested thereby forming an image; and,
a detector arranged proximate to said image for detecting the image.
It is an advantageous effect that the apparatus of the invention perform lens testing at any field angle and many magnifications without the need to build a separate tester for each lens.